All magnetic resonance imaging methods require the following essential elements:
(1) a uniform magnetic field, PA1 (2) controllable magnetic field gradients, PA1 (3) a transmitter/receiver coil system, PA1 (4) an NMR spectrometer, PA1 (5) a data store and processor, PA1 (6) an image display system, and PA1 (7) a central controller.
The transmitter/receiver coil system, often called the "probe", provides the means by which rf energy is transmitted to the spin system and through which the rf response to the spin system is detected. In many applications, a single coil with appropriate coupling can perform both functions. Tuning of the coil is required for both transmitting and receiving functions. Such tuned coil systems may be characterized by their geometry, a quality factor Q, impedance, and size.
It is desirable to maximize the signal-to-noise (S/N) ratio of the NMR signal in every possible way. The S/N ratio attainable with a tuned coil varies as Q.sup.1/2. If these were the only considerations, Q should be made as large as possible. For a receiver coil the other dominant consideration is impedance matching to the receiver. The matching of the receiver coil to the preamplifier in the receiver depends upon the optimal noise impedance of the preamplifier, which should be designed to match available cable impedance. In practice, the length of cable is expected to be as long as possible in NMR system so as to facilitate the arrangement of the transmitter/receiver coil system with respect to the othe system elements. However, the length of the cable is limited by the requirement of matching its impedance to the receiver coil.
The tuning circuit used for the transmitter/receiver coil system shown in FIG. 1 consists of an inductor or coil 2 and resistance 4 which are, respectively, the inherent inductance and resistance of the probe 3. The effect of the matching capacitors 6 and 8 is to modify the signal and noise voltages as they appear at the output terminals of the probe 3.
A first lead of the coil 2 passes through shielding 10 and connects to the non-inverting input of preamplifier 12. The second lead of the coil 2 having capacitor 6 connected thereto, is connected to shielding 10 which is connected to the inverting input of preamplifier 12. A source of positive potential for controlling bias against the variable capacitor 8 is connected through a resistor 14 in parallel to the non-inverting input of the preamplifier 12. The output of the receiver portion including the preamplifier 12, the probe 3, and the tuning circuit 16, provides an analog of the composite NMR signal which is converted to digital form and fed to a computer (not shown) for performing reconstruction NMR imaging of the subject using a Fourier transform. Such NMR imaging techniques are well known and exemplified by U.S. Pat. No. 4,254,778; No. 4,070,611; No. 4,297,637; No. 4,509,011 and No. 4,471,306 incorporated herein by reference.
The function of circuit 16 is to match the impedance of pick-up coil 2 to the high input preamplifier impedance Z(amp), and for the effective transfer of the signal from the pick-up coil 2 to the preamplifier 12, it is necessary to match the input impedance of the coil 2 to the characteristic impedance Zc of the coaxial cable 10. Impedance matching ensures maximum transfer of the NMR signal from pick-up coil 2 to the preamplifier 12. Capacitor 6 is used for impedance matching, and capacitor 8 is used to tune the circuit to the resonant frequency to sweep across the frequency band of interest but has little effect on the input impedance. In order for the combination of capacitors 6 and 8 to have a very high quality factor (Q), the capacitors 6 and 8 are adjusted to make their capacitances approximately equal to facilitate tuning.
However, tuning is nevertheless difficult because of stray capacitance effects of the cable 10, and to facilitate tuning in consideration with the stray capacitance, the length of cable 10 must be limited despite competing practical requirements for a long cable length.
The approach of keeping Q high by adjusting the capacitors 6 and 8 in the previously mentioned manner is unsatisfactory because an electrical coupling exists between the pick-up coil 2 and the patient, which causes Q to be lowered, and this results in a competing consideration on the input impedance of preamplifier 12 and the electrical coupling.
The present invention provides an improved tuning circuit for an NMR transmitter and receiver system without loss of Q and without causing a strict limitation of the cable length. The invention also provides the added benefit of matching the effective pick-up coil impedance to the receiver preamplifier input impedance in an optimum manner. In this fashion, a longer cable is allowed to be used for connection between the pick-up probe and the receiver preamplifier despite its high stray capacitance while providing a high Q for the receiver system consisting of the combination of the pick-up coil, the tuning circuit, the shielding cable, and the receiver preamplifier, and desirably maximizing the signal-to-noise ratio of the NMR signal.